In recent years, there has been increasing demands for secondary batteries as power sources for transportation apparatuses such as electric vehicles, hybrid vehicles and plug-in hybrid vehicles, and also for domestic and commercial large-sized power storage devices. As such power sources, lithium-ion secondary batteries are widely used. In a lithium-ion secondary battery, lithium ions are used as a charge carrier. However, lithium is a rare metal, thus presenting the problems of expensiveness and scarce yield.
As an alternative secondary battery, sodium-ion secondary batteries are under study. In a sodium-ion secondary battery, sodium ions are used as a charge carrier. As compared to lithium, sodium is abundant and is inexpensively available, and thus is drawing attention for a secondary battery that is low-cost and is capable of being implemented in large size. However, there has been conventional wisdom that, even if a material that is deemed usable as a negative-electrode active material in a lithium-ion secondary battery (e.g., carbon materials of highly-graphitized structure, such as graphite) is straightforwardly used as a negative-electrode active material in a sodium-ion secondary battery, it is very difficult to realize a sodium-ion secondary battery having adequate performance (see Patent Document 1). Therefore, in order to realize practical applications of sodium-ion secondary batteries, positive- and negative-electrode materials, and especially a high-capacity negative-electrode material, are being desired and developed.
For example, Patent Document 1 proposes using a carbon material in amorphous glass form as a negative-electrode active material of a sodium-ion secondary battery. This is reported to provide a discharge capacity per unit weight of 265 mAh/g at the most.
On the other hand, Patent Document 2 describes use of hard carbon as a negative-electrode active material of a sodium-ion secondary battery in which an anhydrous electrolytic solution containing a certain electrolytic-solution additive is used. This is reported to provide a discharge capacity per unit weight of about 250 mAh/g at the most.